[llvm] CodeGen: Record tied virtual register operands in finalizeBundle (PR #166209)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle updated https://github.com/llvm/llvm-project/pull/166209

>From f13a14b93c308732c71438788f2d8b0e60ea8687 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicolai=20H=C3=A4hnle?= <nicolai.haehnle at amd.com>
Date: Thu, 18 Sep 2025 16:38:50 -0700
Subject: [PATCH] CodeGen: Record tied virtual register operands in
 finalizeBundle

This is in preparation of a future AMDGPU change where we are going to
create bundles before register allocation and want to rely on the
TwoAddressInstructionPass handling those bundles correctly.

v2:
- simplify the virtual register check and the test

commit-id:64872792
---
 llvm/lib/CodeGen/MachineInstrBundle.cpp     | 22 ++++++++++++++++++++-
 llvm/test/CodeGen/AMDGPU/finalizebundle.mir |  2 +-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/MachineInstrBundle.cpp b/llvm/lib/CodeGen/MachineInstrBundle.cpp
index da29ffc9d2fed..88d81993fbe55 100644
--- a/llvm/lib/CodeGen/MachineInstrBundle.cpp
+++ b/llvm/lib/CodeGen/MachineInstrBundle.cpp
@@ -136,6 +136,7 @@ void llvm::finalizeBundle(MachineBasicBlock &MBB,
   SmallSetVector<Register, 8> ExternUses;
   SmallSet<Register, 8> KilledUseSet;
   SmallSet<Register, 8> UndefUseSet;
+  SmallVector<std::pair<Register, Register>> TiedOperands;
   for (auto MII = FirstMI; MII != LastMI; ++MII) {
     // Debug instructions have no effects to track.
     if (MII->isDebugInstr())
@@ -161,6 +162,15 @@ void llvm::finalizeBundle(MachineBasicBlock &MBB,
           // External def is now killed.
           KilledUseSet.insert(Reg);
         }
+        if (MO.isTied() && Reg.isVirtual()) {
+          // Record tied operand constraints that involve virtual registers so
+          // that bundles that are formed pre-register allocation reflect the
+          // relevant constraints.
+          unsigned TiedIdx = MII->findTiedOperandIdx(MO.getOperandNo());
+          MachineOperand &TiedMO = MII->getOperand(TiedIdx);
+          Register DefReg = TiedMO.getReg();
+          TiedOperands.emplace_back(DefReg, Reg);
+        }
       }
     }
 
@@ -203,7 +213,17 @@ void llvm::finalizeBundle(MachineBasicBlock &MBB,
     bool isKill = KilledUseSet.contains(Reg);
     bool isUndef = UndefUseSet.contains(Reg);
     MIB.addReg(Reg, getKillRegState(isKill) | getUndefRegState(isUndef) |
-               getImplRegState(true));
+                        getImplRegState(true));
+  }
+
+  for (auto [DefReg, UseReg] : TiedOperands) {
+    unsigned DefIdx =
+        std::distance(LocalDefs.begin(), llvm::find(LocalDefs, DefReg));
+    unsigned UseIdx =
+        std::distance(ExternUses.begin(), llvm::find(ExternUses, UseReg));
+    assert(DefIdx < LocalDefs.size());
+    assert(UseIdx < ExternUses.size());
+    MIB->tieOperands(DefIdx, LocalDefs.size() + UseIdx);
   }
 }
 
diff --git a/llvm/test/CodeGen/AMDGPU/finalizebundle.mir b/llvm/test/CodeGen/AMDGPU/finalizebundle.mir
index d2ec1fcbac84f..279f4298e6418 100644
--- a/llvm/test/CodeGen/AMDGPU/finalizebundle.mir
+++ b/llvm/test/CodeGen/AMDGPU/finalizebundle.mir
@@ -40,7 +40,7 @@ name: test_tied
 body: |
   bb.0:
     ; CHECK-LABEL: name: test_tied
-    ; CHECK: BUNDLE implicit-def %0, implicit-def %2, implicit %1:vgpr_32, implicit $mode, implicit $exec {
+    ; CHECK: BUNDLE implicit-def %0, implicit-def %2, implicit %1:vgpr_32(tied-def 1), implicit $mode, implicit $exec {
     ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vgpr_32 = COPY %1:vgpr_32
     ; CHECK-NEXT:   [[V_FMAC_F16_e32_:%[0-9]+]]:vgpr_32 = V_FMAC_F16_e32 internal [[COPY]], internal [[COPY]], %1:vgpr_32, implicit $mode, implicit $exec
     ; CHECK-NEXT: }